Ground Based Augmentation Systems (GBAS) for a navigation satellite systems (NSS) broadcast one or more parameters that can be received by a NSS receiver to correct for various errors in the satellite signals received. One such broadcast parameter is the Vertical Ionosphere Gradient standard deviation, also referred to as sigma-vig (σvig). σvig is used to protect against errors caused by anomalous ionosphere gradients in a Local Area Augmentation System (LAAS).
Typically, σvig is calculated for a future time based on the satellites that will be in view of the GBAS at the future time. Since satellites orbit the earth twice each sidereal day, over time, different satellites rise and set from the perspective of the GBAS. On every cycle, the calculation of σvig is performed for a subsequent time epoch, the next time interval in the future, for all predicted satellites which will be in view of the GBAS at the future time on all predicted sub-geometries. This calculation of σvig is done considering both the maximum horizontal distance (ddg_max) from the LAAS Ground Facility (LGF), which is the GBAS reference location, to the decision height and the maximum horizontal distance (ddh_max) from the decision height to the user (aircraft). This real time geometry screening is applicable for protecting all approaches at an airport.
The larger of the values between the σvig calculated for one time step in the future, and the σvig value previously computed for what is now the current time step is broadcast to the NSS receivers. In some implementations, σvig is calculated at 1 minute intervals to minimize the occurrence of having more than one satellite rise and set in one time increment. At each interval, it calculates a σvig value. The σvig value that will be broadcast is the maximum between the most recently calculated σvig and the σvig that was calculated the previous minute.